High side switching of d.c. electrical circuits is widely used in many applications. In common automotive practice high side switching reduces the number of wires that need to be run to various loads. Only one switched "hot" line is required per load since the return current path is through the shared ground connection.
Mechanical switches and relays normally control the operation of various automotive electrical loads; however, such contact mechanisms are not very reliable when high current or high voltages are switched. Solid state switches or relays have been used at lower current levels for some time where the power losses in the relay are overshadowed by the greater reliability afforded by solid state designs.
At higher current levels, such as those encountered in automobiles, up to 30 Amperes would have to be switched. The nominally 0.7 Volt collector-emitter saturation voltages encountered in bipolar transistor switches would, when controlling a 10 Amp current, causes a transistor dissipation of at least 0.7 V.times.10 A=7 Watts. This heat would have to be dissipated in a rather large heat sink, especially at elevated ambient temperatures.
Newer power MOS-FETS (metal oxide semiconductor-field effect transistors) with saturated "on" resistances (R.sub.DS on) of approximately 0.01 Ohms would allow a 10 A load to be controlled with a dissipative switch (I.sup.2 R) loss of only 1 Watt.
P-channel MOS-FETS are ideally used for high side switching because input biasing and control is simplified. However, the geometry and manufacture of P-channel MOS-FETS produces devices with "on" resistances much higher than equivalent die size N-channel transistors. This intrinsic difference in P and N channel devices is reflected in their cost; the N-channel part being significantly less expensive.
Simple substitution of a P-channel FET by an equivalent N-channel device is not effective. In order to achieve the low forward voltage drop behavior of the field effect transistor in fully saturated mode, the gate voltage must rise above the drain voltage. When N-channel FETS are used for high side switching of a positive supply, a level-shifting circuit is required if the FET is to be effectively switched "on" by a voltage lower than the load supply voltage.
These level-shifting circuits are required because when a high side N-channel FET is turned "on", the source voltage is driven to the positive supply rail. The gate cannot go positive with respect to the drain (connected to the supply rail) because without an added power supply, the gate cannot rise above the source (when the FET is "on"), and genrally V.sub.GS must exceed 5 to 10 Volts minimum for I.sub.DS to be saturated.
The extremely high input resistance of MOS-FET power transistors allows several unique schemes to be used for raising the gate control voltage (V.sub.GS above the load supply voltage). Currently known level-shifting methods are discussed in detail in "High Side Switching With N-Channel MOS-FETS" by J. W. Kerr, Jr. published in the Feb. 7, 1984 issue of Electronic Products.
All known level-shifting means adapted to d.c. high side switching exhibit certain performance problems. Ideally, a MOS-FET switch or relay should emulate the performance of its electromechanical counterpart, i.e., low forward voltage drop and zero off-state current drain. The d.c. switching circuits described by J. W. Kerr, Jr. in the Electronic Products article include a transformerless voltage multiplier circuit where a separate high voltage generator is connected to the gate by auxilliary switching transistors. The standby current of this circuit is very high because the high voltage generator runs all the time.